Many industrial processes, such as coking processes or chemical treatment processes, employ large pressure vessels, often disposed vertically with top and bottom closure mechanisms. The bottom closure usually includes a cover that is generally quite massive and secured to the vessel by dozens of bolts. In order to remove the bottom cover from the vessel (a process sometimes referred to as "unheading" the vessel), the bottom cover is generally first supported by a hydraulic lifting mechanism, then disengaged from the vessel, at which point it can be lowered by the hydraulic mechanism. The operation is reversed to resecure the cover in place. The disengagement and reengagement of the bottom closure can be quite labor intensive, given the mass of the bottom cover and the numerous bolts that hold it in place.
Examples of removable closures are shown in U.S. Pat. Nos. 4,820,384 and 5,290,072, each to Pechacek, and U.S. Pat. No. 5,221,019, to Pechacek et al. For example, the Pechacek '384 patent discloses a remotely operable vessel cover assembly including a cover 10, which can be attached to a flange 5 surrounding an opening in the vessel. When the cover 10 is raised into position, a series of connector pins 60 fits through corresponding holes 20f in a force ring 20 and keyhole-shaped holes 30a in a lock ring 30, which is rotated so that heads 60e of the connector pins 60 are locked behind the lock ring 30. Fluid pressure is then applied to a force actuator 40, pressurizing inner and outer annular rings 40a, 40b, which expand to pre-stress the pins 60 and the cover 10. A ramp ring 50 is then rotated until a series of ramps 50a firmly contacts a complementary series of ramps 10b on the cover 10. The force actuator 40 is then depressurized. According to the Pechacek '384 patent, the angle of inclination of the ramps 50a is sufficiently shallow that friction between the ramp ring 50 and the cover 10 prevents slippage.
The Pechacek '384 patent shows a mechanism for facilitating engagement and removal of the cover 10. Rather than individually tightening and loosening a large number of bolts, connector pins 60 are selectively locked behind a locking ring 30. However, the mechanism disclosed in the '384 patent is complex. For example, in order to secure the cover 10, the mechanism requires first setting the lock ring 30 in position behind unstressed pins 60, then stressing the pins 60 by means of a complex pressure ring assembly, and then utilizing a separate ramp ring 50 to support the pins 60 once they are stressed. Further, as can be seen in FIG. 1 thereof, when the cover 10 is brought into or out of engagement with the vessel, the pins 60 are exposed so as to be susceptible to damage due to contact with the cover 10.
Another example, U.S. Pat. No. 5,228,825, to Fruchtbaum et al., discloses a pressure vessel closure device in which a vessel V is equipped with a head assembly A, which includes a lower flange F at an opening into the vessel V. A generally circular bottom head H (blind flange) is secured against the flange F for sealing the vessel V with a suitable gasket. In normal use, the vessel V is sealed by a plurality of springs 14 and hooks 12. The hooks 12 anchor a movable retaining element 10 to the bottom head H. The springs 14 bias the retaining element 10 away from the flange F to tension the hooks 12 to seal the bottom head H against the flange F.
In the '825 patent, a plurality of cylinders 30 is disposed to draw the movable retaining element 10 downwardly toward the flange F. Before the bottom head H is attached, the hooks 12 are spread, as shown in FIG. 7 of that patent. The cylinders 30 are pressurized to compress the springs 14, the bottom head H is raised to engage the flange F, and the hooks 12 are then pivoted inwardly below forks 36. The cylinders 30 are depressurized, and the springs 14 then bias the retaining element 10 upwardly from the flange F, and the hooks 12, in turn, engage the forks 36 and securely hold the bottom head H in place against the flange F.
Rather than using traditional bolts or pins, the device of the '825 patent utilizes hooks and forks in combination with an axially movable retaining element. The described device also requires that each of the hooks be pivoted in order to engage them behind or disengage them from the forks.
Similarly, U.S. Pat. No. 2,734,824, to DeLuca, shows an autoclave or pressure vessel that includes a vertically disposed tank 10 having a bottom cover. A plurality of bolts 46 is spaced about the tank 10 and arranged to engage a cover 13 when the cover 13 is closed. Each bolt 46 is threadably secured at its upper end to a bifurcated member 47 and has a shoulder 48 which rests tightly against the lower surface of the member 47. Corresponding to each bolt is a radial slot 61' in the edge of the cover and a slot 61 in a flange 65 of the tank 10. The lower end of each bolt 46 is provided with a nut 59 and washer 60 to engage the underside of the cover 13. Cylinders 49 function to move their respective bolts downwardly, and a series of cams 56 cooperates to swing the bolts outwardly to disengage the bolts from the cover and flange to permit opening of the cover 13.
The bolts 46 in the '824 patent, while providing the basic means for holding the cover 13 in position, would not, according to that patent, prevent accidental opening of the cover 13 should the hydraulic pressure on cylinders 49 fail. In order to avoid such an occurrence, a locking ring 64 is provided on top of the flange 65. The upper shoulder 48 of each bolt 46 is spaced slightly above the top surface of the locking ring 64 when the cover 13 is locked closed. The ring 64 has a plurality of rectangularly shaped cut-out portions 66, and one edge of each of these cut-out portions 66 is provided with a narrow slot 67. The ring 64 can be rotated to a locked position in which the slot 67 engages the body of the bolt 46 and is aligned with its upper collar 48, as shown in FIGS. 4 and 5 of the '824 patent. On the other hand, each member 47 may be aligned with its associated cut-out portion 66 when the cover 13 is to be opened.
The apparatus shown in the '824 patent utilizes a relatively complex mechanism that requires the bolts 46 to be pivoted to be seated and unseated. Further, the same hydraulics that actuate the pivoting must also bear the downward force on the bolts when the tank is sealed. Thus, the locking ring must be employed separately to guard against accidental opening in the event of hydraulic failure.
Thus, there is a need in the art for a mechanism that selectively and reliably secures a cover to a flange of a pressure vessel, which can be simply and efficiently engaged and disengaged. There is a further need for such a mechanism that is conducive to remote actuation. There is a still further need for such a mechanism that does not rely on hydraulic pressure to maintain the engagement. There is an additional need in the art for a mechanism that utilizes bolts or studs to selectively secure a cover to a flange of a pressure vessel, wherein the bolts or studs can be retracted when the cover is disengaged from the flange.